This invention relates to waveguide lenses and in particular to a highly efficient broadband waveguide lens for use in very high frequency broadband multiple beam antennas.
Waveguide lenses are used to focus electromagnetic energy into a feed, or into a cluster of feeds, in a manner similar to parabolic mirrors. This type of lens comprises an assemblage of short waveguide elements posistioned side by side, with the combined inner and outer surfaces shaped to a lens contour. Waveguide lenses are in general very lightweight devices. This property makes them preferred over dielectric lenses at radio frequencies. Several varieties of waveguide lenses exist. One variety, the zoned waveguide lens, is made of hollow waveguides and its outer surface is stepped in concentric rings of appropriate radii. Another variety, the constant thickness waveguide lens, employs a phase shifter in each waveguide element to produce the phase correction required for focusing.
The principal advantage of a waveguide lens over a parabolic reflector is the absence of a feed in the path of a wave incident on the lens. This advantage makes the waveguide lens a preferred component in multiple beam antenna and other systems in which relatively large feeds are required. Another advantage of a waveguide lens over a parabolic reflector is its better beam scan characteristics.
The chief disadvantage of waveguide lenses of conventional design is the narrow bandwidth over which the plane wave produced by a lens remains flat within given tolerances. For instance, for a tolerance of .+-.1/16 of a wavelength the bandwidth ranges from about 4% for a lens of diameter equal to 20 wavelengths to about 2% for a lens of diameter equal to 100 wavelengths.
In addition to bandwidth considerations the design of waveguide lenses comprehends both lens thickness and phase error. Waveguide lenses are commonly zoned for either minimum phase error or minimum thickness. It is another disadvantage that zoned waveguide lenses exhibit a reduction in efficiency due to the existance of shadowing effects.
Accordingly these currently exists the need for a waveguide lens that has a wider bandwidth and that is more efficient than state-of-the-art zoned waveguide lenses. The present invention is directed toward satisfying that need.